In percutaneous transluminal coronary angioplasty (PTCA), a balloon catheter is inserted through a brachial or femoral artery, positioned across a coronary artery occlusion, and inflated to compress against atherosclerotic plaque to open, by remodeling, the lumen of the coronary artery. The balloon is then deflated and withdrawn. Problems with PTCA include formation of intimal flaps or torn arterial linings, both of which can create another occlusion in the lumen of the coronary artery. Moreover, thrombosis and restenosis may occur several months after the procedure and create a need for additional angioplasty or a surgical bypass operation. Stents are used to address these issues. Stents are small, intricate, implantable medical devices and are generally left implanted within the patient to reduce occlusions, inhibit thrombosis and restenosis, and maintain patency within vascular lumens such as, for example, the lumen of a coronary artery.
The treatment of a diseased site or lesion with a stent involves both delivery and deployment of the stent. Stent delivery refers to introducing and transporting the stent through an anatomical lumen to a desired treatment site, such as a lesion in a vessel. An anatomical lumen can be any cavity, duct, or a tubular organ such as a blood vessel, urinary tract, and bile duct. Stent deployment corresponds to expansion of the stent within the anatomical lumen at the region requiring treatment. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into an anatomical lumen, advancing the catheter in the anatomical lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen with the stent remaining at the treatment location.
In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon prior to insertion in an anatomical lumen. At the treatment site within the lumen, the stent is expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn from the stent and the lumen, leaving the stent at the treatment site. In the case of a self-expanding stent, the stent may be secured to the catheter via a retractable sheath. When the stent is at the treatment site, the sheath may be withdrawn which allows the stent to self-expand.
Stents are often modified to provide drug delivery capabilities to further address thrombosis and restenosis. Stents may be coated with a polymeric carrier impregnated with a drug or therapeutic substance. A conventional method of coating includes applying a composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer.
The application of a uniform coating with good adhesion to a substrate can be difficult for small and intricate medical devices, such as certain stents for coronary and peripheral arteries. Such stents can be quite small, typically having an overall diameter of only a few millimeters and a total length of several millimeters. Also, such stents are often in the form of a fine network or mesh of thin struts which provide support or push against the walls of the anatomical lumen in which the stent is implanted.
For example, FIG. 14 shows an upper portion of a stent 10 having an overall body shape that is hollow and tubular. The stent can be made from wires, fibers, coiled sheet, with or without gaps, or a scaffolding network of rings. The stent can have any particular geometrical configuration, such as a sinusoidal or serpentine strut configuration, and should not be limited to what is illustrated in FIG. 14. The variation in stent patterns is virtually unlimited. The stent can be balloon expandable or self-expandable, both of which are well known in the art.
FIGS. 14 and 15 show stents with two different stent patterns. The stents are illustrated in an uncrimped or expanded state. In both FIGS. 14 and 15, the stent 10 includes many interconnecting struts 12, 14 separated from each other by gaps 16. The struts 12, 14 can be made of any suitable material, such as a biocompatible metal or polymer. The polymer could also be a bioabsorbable polymer. The stent 10 has an overall longitudinal length 40 measured from opposite ends, referred to as the distal and proximal ends 22, 24. The stent 10 has an overall body 50 having a tube shape with a central passageway 17 passing through the entire longitudinal length of the stent. The central passageway has two circular openings, there being one circular opening at each of the distal and proximal ends 22, 24 of the overall tubular body 50. A central axis 18 runs through the central passageway in the center of the tubular body 50. At least some of the struts 12 are arranged in series to form sinusoidal or serpentine ring structures 20 that encircle the central axis 18.
FIG. 16 is an exemplary cross-sectional view of the stent 10 along line 16-16 in FIG. 15. There can be any number of struts 12, 14 along line 16-16, which runs perpendicular to the central axis 18 of the stent 10. In FIG. 16, the cross-section of seven struts 12, 14 are shown for ease of illustration. The struts 12, 14 in cross-section are arranged in a circular pattern having an outer diameter 26 and an inner diameter 28. The circular pattern encircles the central axis 18. A portion of the surface of each strut faces radially inward in a direction 30 facing toward the central axis 18. A portion of the surface of each strut faces radially outward in a direction 32 facing away from the central axis 18. The various strut surfaces that face radially outward collectively form the outer surface 34 of the stent 10. The various strut surfaces that face radially inward collectively form the inner surface 36 of the stent 10.
The terms “axial” and “longitudinal” are used interchangeably and relate to a direction, line or orientation that is parallel or substantially parallel to the central axis of a stent or a central axis of a cylindrical structure. The term “circumferential” relates to a direction along a circumference of a stent or a circular structure. The terms “radial” and “radially” relate to a direction, line or orientation that is perpendicular or substantially perpendicular to the central axis of a stent or a central axis of a cylindrical structure.
Coating of the thin network of struts often leads to pooling or webbing of the coating substance where struts meet, non-uniform coating thickness and distribution, delamination, contamination. Many spray coating systems are inefficient and produce a high incidence of coating defects due in part to insufficient control of the spray and dry environment.
A coating process may require the application of several coating substances applied separately as a primer layer, a drug carrying reservoir layer, and a top coat or drug diffusion barrier. Each coating layer can involve the use of multiple compounds to form a blend of solvent, polymer, and drug. Also, a coating process may includes multiple spray and dry cycles to form a desired thickness for each coating layer. Thus, it can be difficult to keep track of coating cycles and the types or batches of coating substances for each cycle. Keeping track and recording of such details is important for quality and regulatory control. Since the amount of drug on the stent or the desired properties of each coating is directly proportional to the coating thickness and weight, the unique identity of each stent must be tracked as it progresses down the manufacturing line. To ensure accurate tracking, many systems and methods involve a one-piece flow manufacturing model wherein a spray coating machine processes one stent at a time, which can be inefficient and time consuming because of the time require for drying between coats and because of the need for multiple coats on each stent. An approach to increase manufacturing output would be use several spray coating machines in parallel, as in a multi-piece flow manufacturing scheme. A disadvantage of this approach is that it deviates from the one-piece flow manufacturing scheme that controls stent identity in a highly reliable way and, thus, may allow stents to become mixed up from time to time due to loss of tracking identity. Loss of tracking identity causes a stent, or even an entire production lot of stents, to be scrapped to waste.
Another difficulty in producing drug-coated medical devices, such as drug eluting stents, is that the drugs, solvents, and other substances used in the manufacturing process can be dangerous to the health of human operators of manufacturing equipment. In some cases, the drug can be an immunosuppressant, which can have a significant effect even in very small amounts not noticeable by normal smell or sight.
Accordingly, there is a continuing need for a system and a method for coating medical devices that are efficient, reliable, and take into account the health and safety of persons involved in the manufacturing process.